Electronic blood pressure meter

ABSTRACT

Disclosed is an electronic blood pressure meter which is adapted to measure blood pressure from a finger. A cuff for applying pressure to the finger comprises a plurality of chambers which are communicated to each other and, thereby, provides a sufficient flexibility to closely conform to the contour of the finger. A sensor for detecting pulse wave data may be conveniently place on the inner surface of the cuff and is adapted to detect the reflection of light by an artery. Further, an air buffer can increase the effective air volume of the cuff and can reduce the rate of the pressure drop of the air cuff for a given rate of air vent from the cuff. By deriving the pulse wave data and computing the systolic and diastolic blood pressure therefrom in the course of increasing the air pressure of the cuff, the discomfort to the patient whose blood pressure is to be measured can be substantially reduced.

This application is a divisional of Ser. No. 850,819, filed Apr. 11,1986.

TECHNICAL FIELD

The present invention relates to an electronic blood pressure meter formeasuring the blood pressure of a living body by applying pressure to apart of the living body and detecting the resulting change in the volumeof the artery and in particular to an electronic blood pressure meterwhich is adapted to measure the blood pressure of an artery of a fingerof a living body.

BACKGROUND OF THE INVENTION

Conventional electronic blood pressure meters are mostly based on anindirect method in which a cuff is wrapped around an upper arm of apatient and pressurized to obstruct the blood flow in the upper arm, andthe cuff pressures corresponding to the time points of appearance anddisappearance of Korotkoff sound during the process of depressurizingthe cuff are determined as the systolic (maximum) blood pressure and thediastolic (minimum) blood pressure, respectively.

An electronic blood pressure meter based on Riva-Rocci-Korotkoff methodhowever has the disadvantage that since the Korotkoff sound is to bepicked up by a microphone an accurate measurement of blood pressure issometimes impossible particularly when the surrounding is noisy or whenthe cuff is rubbed by an object and the resulting sound is picked up bythe microphone.

According to another method of measuring blood pressure or so-calledoscillation method, the pulse wave produced in a living body insynchronization with the pumping motion of a heart is measured and bloodpressure values are computed using the amplitude of the pulse wave as aparameter according to a certain algorithm.

Since this method does not require a microphone to pick up the pulsewave from an artery, the above-mentioned problems of theRiva-Rocci-Korotkoff method would not occur but the oscillation methodstill requires to apply a cuff to one's upper arm and it is quitecumbersome that the patient must roll up his sleeve for blood pressuremeasurement.

In view of such inconvenience of the prior art, the present inventorshave realized that the above mentioned problems will be eliminated if anaccurate blood pressure measurement can be performed on a part of a bodywhich is normally exposed, such as a finger.

A certain device is known according to which water is used for pressinga finger for the purpose of measuring blood pressure from an artery inthe finger, but a roller pump is necessary for the pressure control ofthe water which fills a cuff for the application or pressure to thefinger and must be provided separate from the main unit, making itimpossible to achieve a desired compactness of the structure.

Electronic blood pressure meters using air cuffs for pressurizing one'supper arm are well known in the art but an electronic blood pressuremeter using such a cuff can not be directly applied to measuring bloodpressure by one's finger since the volume of the cuff and the ventingspeed are excessive and the pressurization unit and, therefore, thesignal processing unit of a conventional electronic blood pressure meterare unsuitable for this application.

For instance, a typical air cuff for a conventional blood pressure meterconsists of a rectangular flexible air bag having an outer and an innerskin having the same dimensions and, therefore, when it is wrappedaround one's arm and inflated by air pressure, the inner skin tends todevelop creases or folds thereby causing uneven application of pressureto the upper arm. This tendency becomes more pronounced as the cuff iswrapped around an object having a smaller diameter such as a finger.Furthermore, the orientation of a sensor device attached to the innerskin tends to be unpredictable if such folds are produced in thevicinity of the sensor, thus reducing the reliability of the sensor.

In measuring blood pressure, it is necessary to detect the pulse wave ofan artery but the artery in one's finger is so fine that a conventionalpulse wave detector is not adequate for accurate detection. In aconventional pulse wave detector, as shown in FIG. 6, light emitted froma light emitting element L (for instance an LED) through an artery D ina finger F is received by a light sensitive element PT (for instance aphoto transistor) and the pulse wave of the artery is detected as thechanges of the intensity of the light received by the light sensitiveelement PT. According to this detector, since the light must passthrough a distance corresponding to the width of the finger, it isdifficult to achieve a desired sensitivity and the signal to noise ratio(SN ratio) of the signal detected by the light sensitive element PTtends to be low.

Another shortcoming of the above-mentioned conventional methods ofmeasuring blood pressure is that since the measurement process takesplace as the air pressure is gradually reduced and a substantialpressure must be built up in the cuff prior to starting the measurementthe patient is subjected to a discomfort for a substantial time period.

BRIEF SUMMARY OF THE INVENTION

In view of such shortcomings of the prior art, a primary object of thepresent invention is to provide an electronic blood pressure meter whichis accurate and easy to use.

Another object of the present invention is to provide an electronicblood pressure meter which is adapted to be used on a finger of a livingbody.

Yet another object of the present invention is to provide an electronicblood pressure meter which can achieve an extremely slow venting from acuff so as to allow measurement of blood pressure by a finger of aliving body.

Yet another object of the present invention is to provide an electronicblood pressure meter which can measure blood pressure as a cuff is beingpressurized and thus can reduce the discomfort of the patient.

Yet another object of the present invention is to provide anadvantageous air cuff for an electronic blood pressure meter which isadapted to be applied to a part of a living body having a relativelysmall diameter such as a finger.

Yet another object of the present invention is to provide anadvantageous pulse wave detector for an electronic blood pressure meterwhich is highly sensitive and can achieve a high SN ratio.

According to a broad concept of the present invention, such objects areaccomplished by providing an electronic blood pressure meter formeasuring blood pressure, comprising: a cuff made of flexible materialand defining an air chamber therein for applying air pressure to afinger inserted in a cylindrical space defined by an internal surface ofthe cuff; pressure control means connected to the air chamber defined inthe cuff for varying the air pressure inside the cuff; a cuff pressuresensor for detecting the air pressure in the air chamber of the cuff;pulse wave information detecting means attached to the cuff fordetecting pulse wave information as the air pressure in the air chamberof the cuff is varied; and blood pressure determination means fordetermining a blood pressure value from the cuff air pressure detectedby the cuff pressure sensor and the pulse wave information detected bythe pulse wave information detecting means.

According to a certain aspect of the present invention, the pressurecontrol means comprises an air pump, a pressure sensor, a fast ventvalve and a slow vent valve. And, additionally, an air buffer may beconnected either directly or indirectly to the air cuff for the purposeof increasing the effective volume of the air cuff and reducing the rateof pressure drop in the air cuff for a given venting rate. Byappropriate arrangement of these pressure control elements, it ispossible to measure blood pressure values from the pulse wave datadetected by the pulse wave detecting means either while the cuffpressure is being increased or while the cuff pressure is being reduced.

According to another aspect of the present invention there is provided acuff for measuring blood pressure by surrounding a finger of a livingbody and obstructing the blood flow in the finger, comprising: an airbag having an internal and an external skin which have a certain lengthand a certain width sufficient for substantially surrounding the finger,the interior of the air bag defined by the two skins being divided intoa plurality of air chambers, which are communicated with each other,along a circumferential direction of the air bag as it surrounds thefinger, the internal skin of the air bag adjacent to the finger beingsubstantially flexible at least along the longitudinal direction of thefinger and being adapted to be inflated individually with respect todifferent parts of the internal skin which is adjacent to the fingerdefining different ones of the divided air chambers; a pulse wave sensorattached to the internal skin of the air bag; and a conduit provided inthe air bag for supplying and venting air pressure into and from the airchambers.

It is particularly preferable if the inner skin is provided with aplurality of bulges of a substantially trapezoidal or semicircular shapedefining the corresponding air chambers. Thereby, when the cuff iscurved into a cylindrical shape, the top surfaces of the bulges define acylindrical space for inserting a part of a living body such as a fingertherein while the lateral side portions of the bulges cme closertogether without interfering each other. As a result, a uniform contactbetween the air cuff and a part of a living body for instance a fingercan be achieved and high measurement accuracy can be assured.

According to yet another aspect of the present invention, there isprovided a pulse wave detector for detecting the pulsation of an arteryin a living body; comprising: a light emitting element for emittinglight onto an artery of a living body; and a light sensitive element fordetecting the reflection of the light from the light emitting elementand producing a signal representative of the light received by the lightsensitive element. According to this aspect of the present invention,since the path of the light in the living body is generally shorter thanthat of the conventional pulse wave detector according to which light istransmitted through a living body for the detection of pulse waves, theattenuation of the light is less and higher detection sensitivity can beachieved.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be shown and described in the followingin terms of concrete embodiments thereof with reference to the appendeddrawings, in which:

FIG. 1(a) is a sectional view of a cuff assembly according to thepresent invention illustrating how the pulse wave data can be obtainedfrom an artery in a finger;

FIG. 1(b) is a perspective view of the cuff assembly of FIG. 1(a);

FIG. 2 is partially broken away perspective view of an embodiment of theelectronic blood pressure meter according to the present invention;

FIG. 3 is a perspective view of an embodiment of the cuff according tothe present invention in its developed state;

FIG. 4 is a schematic block diagram of a first embodiment of theelectronic blood pressure meter according to the present invention;

FIG. 5 is a wave form diagram comparing the outputs from a pulse wavedetector according to the present invention and a conventional pulsewave detector;

FIG. 6 is a schematic sectional view of an example of conventional pulsewave detector;

FIG. 7 is a sectional view of the cuff shown in FIG. 3;

FIGS. 8 to 10 are sectional views similar to FIG. 7 showing differentembodiments of the cuff according to the present invention;

FIG. 11 is a flow chart illustrating the action of the first embodimentof the electronic blood pressure meter according to the presentinvention;

FIG. 13 is a flow chart illustrating the action of a second embodimentof the electronic blood pressure meter according to the presentinvention;

FIG. 14 is a schematic block diagram of a third embodiment of theelectronic blood pressure meter according to the present invention;

FIG. 15 shows graphs of the cuff pressure and the pulse wave dataaccording to the third embodiment of the electronic blood pressure meteraccording to the present invention; and

FIG. 16 shows graphs of the cuff pressure and the pulse wave dataaccording to the third embodiment of the electronic blood pressure meteraccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1a and 1b show an embodiment of a pulse wave detector which isinstalled in a cuff unit which will be described in greater detailhereinafter. As shown in these drawings, a substantially cylindricalcuff 2 is fitted into a tubular member 1 and a light emitting element 3such as an LED and a light sensitive element 4 such as a phototransistor are attached, adjacent to each other, in the internal wall ofthe cuff 2 curved into a cylindrical shape.

As shown in FIG. 1a, a finger 5 is inserted in the interior of thecylindrical cuff 2 which is adapted to be pressed onto the finger 5 bybeing pressurized by an air pump in a manner which is described ingreater detail hereinafter. The light from the light emitting element 3is projected onto an artery 6 of the finger 5 and, after being reflectedby the artery 6, reaches the light sensitive element 4. Therefore, thelight sensitive element 4 can detect the pulse wave of the artery 6 asthe changes in the intensity of the light received thereby while thecuff 2 is being pressurized or depressurized.

According to the illustrated embodiment, since the overall length of thelight path between the light emitting element 3 and the light sensitiveelement 4 by way of the artery 6 is shorter than that of the prior artaccording to which the light passes through a finger as describedearlier with reference to FIG. 6, the pulse wave level detected by theembodiment shown in FIGS. 1a and 1b is generally greater in amplitudethan that described by the conventional pulse wave detector illustratedin FIG. 6 typically by a factor of 10 or higher and this difference isclearly indicated in the graph given in FIG. 5 in which (a) denotes thepulse wave level detected by the convention detector while (b) denotesthe pulse wave level detected by the embodiment of the presentinvention.

FIG. 2 shows a partially broken away external view of an embodiment ofthe electronic blood pressure meter according to the present invention.A main body casing 11 accommodates a battery unit 12 located in a righthand rear portion thereof, an air pump 13 located in front of thebattery unit 12, an air buffer 14 located substantially in the middle ofthe casing 11, and a cuff unit 15 located in a left hand portion of thecasing 11. Further, a circuit board 16 extends over the top surfaces ofthe air buffer 14, the battery unit 12 and the air pump 13. A powerswitch 17, a start switch 18 and a liquid crystal display unit 19 areattached to the top surface of the circuit board 16 and appropriateelectrical connections are made therebetween although it is not shown inthe drawing. Also an MPU and other electronic component parts aremounted to the lower surface of the circuit board 16 and this is alsonot shown in the drawing.

The cuff unit 15 includes the tubular member 1 and the cuff 2 which weredescribed earlier with reference to FIGS. 1a and 1b. As shown in FIGS. 3and 7, the cuff 2 comprises an upper skin 9 and a lower skin 10 defininga chamber therebetween and these skins may also be called as an internalskin and an external skin because of their positions when the cuff iscurved into the cylindrical shape shown in FIGS. 1a, 1b and 2. The spacedefined between the two skins 9 and 10 are divided into a plurality ofair chambers 8, which are communicated to each other, with the internalskin 9 formed with so many bulges 23a to 23h which have a relativelysmall width and extend substantially the whole length of the cuff 2. Anair tube 24 is connected to an end surface of one of the bulges 23e forsupplying and venting air into and from the air chambers defined in thecuff 2.

Depressions 25 and 26 are formed in two of the protrusions 23c and 23ddefining a pair of flat surfaces, and the light emitting element 3 andthe light sensitive element 4 are mounted on these flat surfaces. Thelight emitting element 3 comprises a casing 3a made of synthetic resinwhich has a window on top surface thereof as shown in FIG. 3 forpermitting therethrough the passage of light emitted from an LED (notshown in the drawings) received inside the casing 3a. The longitudinalexternal end portion the casing 3a is opened for passing thereinto apair of lead wires 28 for the LED. The light sensitive element 4 has asimilar structure which is likewise provided with a pair of lead wires29 which are however connected to a photo transistor (not shown in thedrawings) provided therein instead of the LED. The lead wires 28 and 29leading out from these elements 3 and 4 are connected to the circuitryprovided in the circuit board 16.

Thus, as described previously with reference to FIGS. 1a and 1b, thelight emitted from the light emitting element 3 is projected onto anartery 6 in the finger 5 through the skin and the flesh of the finger 5is reflected by the artery 6 back to the light sensitive element 4 againthrough the flesh and the skin of the finger 5 so that the pulse wavecan be detected by the light sensitive element 4 as a variation of theintensity of the reflected light.

FIG. 4 shows a functional block diagram of the first embodiment of theelectronic blood pressure meter. As shown in this drawing, the lightemitting element 3 deriving electric power from an MPU 33 is placed onthe cuff 2 adjacent to the light sensitive element 4 whose output issupplied to the MPU 33 by way of an amplifier 31 and an AD converter 32.An air conduit 40 is connected to the cuff 2, and this air conduit 40 isconnected to various forms of air control equipment operative to controlthe air pressure of the air cuff 2; i.e., a pressure sensor 34, an airbuffer 14 or an air accumulator consisting of an air chamber of acertain volume, a slow vent valve 37 and a fast vent valve 38 which aredirectly connected to the conduit 40, and an air pump 13 whose outputend is connected to the conduit 40 by way of one-way valve 36.

The cuff pressure signal detected by the pressure sensor 34 is amplifiedby an amplifier 35 and supplied to the MPU 33 by way of the AD converter32. The MPU 33 is internally equipped with memory for storing a programand values derived during the process of arithmetic operations given bythe program and, through appropriate switching of the AD converter 32,can perform the functions of inputting pulse wave data and cuff pressuredata thereinto, turning on and off the air pump 13, opening and closingthe fast vent valve 38, determining blood pressure from the pulse wavedata, and indicating the status whether measurement is in progress ornot.

The determined blood pressure values or the systolic blood pressure(SYS) and the diastolic blood pressure (DIA) are outputted from the MPU33 and displayed on the display unit 19, and a buzzer 39 may beactivated by a command from the MPU 33 at an appropriate timing as willbe described hereinafter. The air buffer 14 has a certain volume whicheffectively increases the effective volume of the air cuff 2. Because ofthe presence of the air buffer 14, the drop in the pressure of the aircuff 2 is slower for a given rate of air venting as compared to the casein which the air buffer is not provided. As a result, even when a slowvent valve 37 for an arm cuff is applied to the present embodiment usingthe air cuff 2 for a finger, the pressure drop of the air cuff 2 is asslow as 2 to 3 mmHg/sec.

FIG. 7 shows the air cuff 2, used in the first embodiment of the presentinvention, in cross section. The outer skin 10 is made of relativelyhard material but is flexible enough to be deformed from a flat shape toa cylindrical shape of a desired diameter. The inner skin 9 is made offlexible material such as rubber and formed with a plurality of bulges23a to 23h (FIG. 3) which extend substantially the whole length of thecuff 2 and are arranged along the inner circumference of the cuff 2,when it is curved into the cylindrical shape, at substantially equalintervals. The circumferential edges of the inner skin 9 and the outerskin 10 are bonded together.

Each of the bulges 23a to 23h in the inner skin 9 has a substantiallytrapezoidal cross section and comprises a middle flat portion 9a and apair of sloping portions 9b rising towards the lateral side edges of themiddle flat portion 9a. Thus, a plurality of air chambers 8 are definedbetween the inner skin 9 and the outer skin 10 but they are communicatedto each other. And the air tube 24 is connected to an end surface of theinner skin 9 (FIG. 3) for inflating and venting the cuff 2. Two of thebulges 23c and 23d are provided with the depressions 25 and 26 whichdefine the flat surfaces for mounting the light emitting element 3 andthe light receiving element 4 thereon as mentioned previously withreference to FIG. 3.

Thus, the cuff 2 is rolled into the cylindrical shape and fitted intothe tubular member 1 in the electronic blood pressure meter. As aresult, a cylindrical space is defined by the flat top portions 9a ofthe bulges 23a to 23h of the inner skin 9, and adjacent ones of thesloping portions 9b come close together without causing any substantialstrain to the inner and outer skin 9 and 10. After a finger, forinstance a first finger, is inserted into the thus defined cylindricalspace and the cuff 2 is inflated through pressurized air supplied fromthe air tube 9 by turning on the power switch 17 and activating the airpump 13, the flat portions 9a of the bulges 23a to 23h of the inner skin9 can conform to the contour of the finger 5 and will uniformly applypressure to the finger 5.

FIGS. 8 to 10 show different embodiments of the air cuff 2a to 2caccording to the present invention. The embodiment shown in FIG. 8 issimilar to the embodiment given in FIG. 7 but an air tube 24 is providedin the outer skin 10 instead of the inner skin 9. According to theembodiment given in FIG. 9, the bulges in the inner skin 9 are semicircular in cross section, as opposed to the trapezoidal shape of theprevious embodiments, while the inner skin 10 is flat in the same manneras the previous embodiment. As a result, the air chambers 8 have semicircular cross sections. According to the embodiment given in FIG. 10,not only the inner skin 9 is provided with bulges similar to those ofthe embodiment of FIG. 9, but also the outer skin 10 is provided withsimilar bulges 21a to 21h in alignment with the corresponding bulges ofthe inner skin 9. Therefore, the air chambers 8 in this case aresubstantially circular in cross section.

Now the action of the first embodiment of the electronic blood pressuremeter is described in the following particularly with reference to theflow chart given in FIG. 11.

Prior to starting a measurement process, a patient puts his first fingerof his left hand into the cylindrical space of the cuff 2, and the powerswitch 17 is turned on. This starts off the execution of the programstored in the memory of the MPU 33 and performs a segment check for thedisplay unit 14 by turning on all the segments in the display unit 14for 1.5 seconds in step 1. Thereafter, all the segments are turned offin step 2. In step 3, it is determined whether the cuff pressure is zeroor not. If the cuff pressure is not zero, the fast vent valve 10 isactivated in step 22 and the system flow returns to step 3. And, this isrepeated until the cuff is sufficiently deflated. When the cuff pressureis zero in step 3, a ready mark (indicated as a heart shaped mark in thedisplay unit 19 as shown in FIG. 4) is turned on in step 4 and thebuzzer 15 is activated for four consecutive short time intervals in step5. This completes the preparation of the electronic blood pressure meterfor the actual measurement process.

At this instance, if the start switch 18 is turned on, the determinationresult of step 6 becomes yes, and the air pump 13 is activated. In step7, the air pressurized by the air pump 13 is conducted to the mainconduit 40 by way of the one-way valve 36 and is supplied to the airbuffer 14 and the cuff 2 until the air pressure detected by the airpressure sensor 34 reaches a certain value P_(set) programmed in the MPU33. Typically, the value P_(set) programmed in the MPU 33 is higher thanthe expected systolic blood pressure by 20 to 30 mmHg. Then, the airpump 13 is deactivated and venting from the cuff 2 takes place onlythrough the slow vent valve 37 in step 8. However, since the referencelevel for pulse wave detection may not be stable immediately after thecompletion of the pressurization of the cuff 2, a stabilization processis conducted in steps 9 and 10. It is determined in step 9 whether thereference level is stable or not. If not, it is determined whether thepressure level is below the predetermined by value by more than 40 mmHgor not. If so, the cuff is rapidly vented by the fast vent valve 38 instep 21 and the process flow returns to step 3. If the pressure is notlower than the predetermined value by more than 40 mmHg, the processflow returns to step 9. Once the reference level for pulse wavedetection has been stabilized, the amplitude of the pulse wave isdetermined in the subsequent steps.

In step 11 a pulse wave number j corresponding to the part of the pulsewave at which the amplitude is to be determined is set to zero. In step12 the count of a sample counter for one cycle of pulse beat is reset to1 and the pulse wave number is incremented by one. At the same time, amaximum pulse wave level x_(max) is set to zero and a minimum pulse wavelevel x_(min) is set to an upper limit value x_(sup) which is higherthan a conceivable maximum value of the pulse wave level.

In step 13, it is determined whether the cuff pressure is higher than 20mmHg or not. If the cuff pressure is lower than 20 mmHg, the processflow advances to step 21 and rapid venting of the cuff 2 takes place. Ifthe cuff pressure is higher than 20 mmHg, pulse wave data x₁ is inputtedin step 14 and the difference between the current value of the pulsewave data x_(i) and the previous value of the pulse wave data x_(i-1)are compared with a certain value x_(t) in step 15. In step 15 it isdetermined if this difference is greater than this certain value x_(t)or not.

If the determination result of step 15 is negative, the process flowadvances to step 23 and the current value of the pulse wave data x_(i)is compared with the maximum pulse wave level x_(max). If x_(i) >x_(max)the maximum pulse wave level x_(max) is updated by the value of x_(i) instep 24. Conversely, if x_(i) <x_(max) step 24 is skipped and theprocess flow advances to step 25. In step 25 the current pulse wave datax_(i) is compared with the minimum pulse wave level x_(min). If x_(i)<x_(min) the minimum pulse wave level x_(min) is updated by the value ofx_(i) in step 16. Conversely, if x_(i) >x_(min) step 26 is skipped andthe process flow advances to step 27.

When the updating of the maximum and minimum pressure level x_(max) andx_(min) in steps 23 to 26 is completed, the count i is incremented byone in step 27 and the process flow returns to step 13. Thereafter,steps 23 to 27 and steps 13 to 15 are repeated and the updating of themaximum and minimum pulse wave level x_(max) and x_(min) continues untilx_(i-1) -x_(i) >x_(t) or until the next cycle of pulse beat begins.

If it is determined that x_(i-1) -x_(i) >x_(t) in step 15, the buzzer 39is activated in step 16 and extraction of the pulse wave is notified tothe user. Then, the difference between the maximum and minimum pulsewave level x_(max) and x_(min) (the amplitude of the pulse wave) A_(j)is derived in step 17 and stored in the MPU 33. And a blood pressuredetermination process is conducted in step 18 using this amplitude A_(j)as a parameter.

It is possible to determine blood pressure in a number of ways and,according to the present embodiment, the cuff pressure at the time pointt₁ at which the pulse wave starts appearing is determined as thesystolic blood pressure P_(sys) (refer to FIG. 12) and the cuff pressureat the time point t₂ at which the amplitude of the pulse wave A_(j)maximizes is determined as the average blood pressure P_(mean). And thediastolic blood pressure P_(dia) is determined by the followingequation:

    P.sub.mean =P.sub.dia +(P.sub.sys -P.sub.dia)/3

And, until the diastolic blood pressure P_(dia) is determined, theprocess flow continues returning from step 19 to step 12 and, afterincrementing the pulse wave number j by one in step 12, the bloodpressure determination process is continued by deriving the differencebetween the maximum and minimum pulse wave level x_(max) and x_(min) orthe amplitude of the pulse wave) A_(j).

Upon completion of the determination of the the maximum and minimumpressure level (step 19), these blood pressure values are displayed onthe display unit 19 in step 20 and, thereafter, the fast vent valve 38is activated in step 21 to complete the measurement process.

FIG. 13 is a flow chart illustrating the action of another embodiment ofthe electronic blood pressure meter of the present invention. Accordingto the present embodiment, steps 1 and 2 are identical to steps 1 and 2of the previous embodiment, but the fast vent valve 38 is activated oropened in step 3 and the fast vent mark is displayed in the display unit19 in step 4 although it is not shown in the drawings. In step 5, it isdetermined whether the cuff pressure is zero or not and the process flowgoes into a loop until the cuff pressure drops to zero.

When the cuff pressure has dropped to zero, the fast vent mark is turnedoff in step 6 and a ready mark (indicated as a heart shaped mark in thedisplay unit 19 shown in FIG. 4) is turned on in step 7. Then, the usercan know that the electronic blood pressure meter is ready formeasurement. Here, by turning on the start switch 18, the process flowadvances from step 8 to step 9 and an LED on the display unit 19 (notshown in the drawings) for indicating that a measurement process is inprogress is lighted in step 10. Further, the fast vent valve 38 isclosed in step 11 followed by the activation of the air pump 13, and thepressurization of the cuff 2 by the air pump 13 is continued until thecuff pressure reaches a certain pressure P_(set) in step 12.

If this pressurization is determined as being a repressurization processfor resuming a pressurization after an interruption of a pressurizationin step 13 as is described hereinafter, and a repressurization markwhich is described hereinafter is turned off in step 14. In either case,upon completion of the pressurization process, a slow venting is startedin step 15. As mentioned previously, this venting process may beperformed at the rate of 2 to 3 mmHg/sec. Thereafter, the process flowgoes into a loop by way of steps 16 and 17 until the reference level forpulse wave detection is stabilized. If the cuff pressure has droppedbelow the predetermined pressure P_(set) by more than 30 mmHg duringthis process, the repressurization mark is displayed on the display unit19 in step 20 although it is not shown in the drawings and the systemflow returns to step 12 for repressurization of the cuff 2.

At any rate, when it is determined in step 16 that the reference levelfor the pulse wave detection has been stabilized, the process flowadvances to step 21. In step 21 a pulse wave number j corresponding tothe part of the pulse wave at which the amplitude is to be determined isset to zero. In step 22 the count of a sample counter for one cycle ofpulse beat is reset to 1 and the pulse wave number is incremented byone. At the same time, a maximum pulse wave level x_(max) is set to zeroand a minimum pulse wave level x_(min) is set to an upper limit valuex_(sup) which is higher than a conceivable maximum value of the pulsewave level.

In step 23, it is determined whether the cuff pressure is higher than 20mmHg or not. If the cuff pressure is lower than 20 mmHg, the processflow advances to step 31 and, after turning off the LED indicating thatthe measurement process is in progress in step 31, rapid venting of thecuff 2 takes place in step 3. If the cuff pressure is higher than 20mmHg, pulse wave data x_(i) is inputted into the MPU 33 in step 24 andthe difference between the current value of the pulse wave data x_(i)and the previous value of the pulse wave data x_(i-1) are compared witha certain value x_(t). In step 25 it is determined if this difference isgreater than this certain value x_(t) or not.

If the determination result of step 25 is negative, the process flowadvances to step 32 and it is determined whether the pulse wave number jhas not been updated for 1.5 second. If the pulse wave number j has beenupdated during that time interval, the current value of the pulse wavedata x_(i) is compared with the maximum pulse wave level x_(max) in step33. If x_(i) >x_(max) the maximum pulse wave level x_(max) is updated bythe value of x_(i) in step 34. Conversely, if x_(i) <x_(max) step 34 isskipped and the process flow advances to step 35. In step 35 the currentpulse wave data x_(i) is compared with the minimum pulse wave levelx_(min). If x_(i) <x_(min) the minimum pulse wave level x_(min) isupdated by the value of x_(i) in step 36. Conversely, if x_(i) >x_(min)step 36 is skipped and the process flow advances to step 37.

When the updating of the maximum and minimum pulse wave level x_(max)and x_(min) in steps 32 to 36 is completed, the count i is incrementedby one in step 37 and the process flow returns to step 23. Thereafter,steps 32 to 37 and steps 23 to 25 are repeated and the updating of themaximum and minimum pulse wave level x_(max) and x_(min) continues untilx_(i-1) -x_(i) >x_(t) holds or until the next cycle of pulse beatbegins.

If it is determined that x_(i-1) -x_(i) >x_(t) in step 25, the buzzer 39is activated in step 26 and extraction of the pulse wave is notified tothe user. Then, the difference between the maximum and minimum pulsewave level x_(max) and x_(min) (the amplitude of the pulse wave) A_(j)is derived in step 27 and stored in the MPU 33. And a blood pressuredetermination process is conducted in step 28 using this amplitude A_(j)as a parameter. The blood pressure values may be obtained in step 29 inthe same manner as described with reference to FIG. 11.

Upon completion of the determination of the the maximum and minimumpressure level (step 29), the obtained blood pressure values aredisplayed on the display unit 19 in step 30 and, when the process flowhas returned to step 3, the fast vent valve 38 is activated in step 3 tocomplete the measurement process and get ready for the next measurement(steps 6 and 7).

According to the present embodiment, the air buffer 14 was provided inthe casing 21, but it is also possible to provide an air buffer in partof the air cuff 2 itself. Alternatively, it is also possible toeliminate the air buffer 14 as long as a desired rate of venting can beachieved with the slow vent valve 41.

FIG. 14 shows yet another embodiment of the present invention which isbased on a different algorithm for determining the various bloodpressure values. In this drawing, the parts corresponding to those inFIG. 4 are denoted by like numerals and their detailed description isomitted.

According to this embodiment, the outlet of an air pump 13 is connectedto a conduit 45 by way of a one-way valve 36, and the conduit 45 isconnected to another conduit 44 leading to an air cuff 2 by way of aslow vent valve 41. An air buffer 14 and a fast vent valve 43 areconnected to the conduit 45 on the side of the air pump 13 while apressure sensor 34 and another fast vent valve 42 are connected to theother conduit 44 on the side of the air cuff 2.

FIG. 15 is a flow chart illustrating the action of the embodiment shownin FIG. 14.

Prior to starting a measurement process, a patient put his first fingerof his left hand into the cylindrical space of the cuff 2, and the powerswitch 17 is turned on. This starts off the execution of the programstored in the memory of the MPU 33 and performs a segment check for thedisplay unit 14 by turning on all the segments in the display unit 14for 1.5 seconds in step 1. Therafter, all the segments are turned off instep 2. In step 3 the fast vent valves 42 and 43 are opened and in step4 a vent mark (not shown in the drawings) indicating that venting istaking place is lighted. Then it is determined whether the cuff pressureis zero or not in step 5, and the process flow goes into a loop untilthe cuff pressure drops to zero. When the cuff pressure has dropped tozero, the vent mark is turned off in step 6. Thereafter, a ready mark(indicated as a heart shaped mark in the display unit 19 as shown inFIG. 14) is turned on in step 7 and the system flow goes into a loopuntil the start switch 18 is turned on in step 8.

When the start switch 18 is turned on, the ready mark on the displayunit 19 is turned off in step 9 and an LED indicating that themeasurement process is in progress is lighted in step 10. Then, both thefast vent valves 42 and 43 are closed and the air pump 13 is activateduntil the air pressure in the air buffer 14 reaches 300 mmHg in step 11.This pressure is gradually supplied to the air cuff 2 by opening theslow vent valve 41 which leads to the conduit 44 and the cuff 2 in step12. As a result, the pressure in the cuff 2 starts rising as shown inFIG. 15 and at the same time the pulse wave detector 26 starts detectingthe pulse wave.

In step 13 a pulse wave number j corresponding to the part of the pulsewave at which the amplitude is to be determined is set to zero. In step14 the count i of a sample counter for one cycle of pulse beat is resetto 1 and the pulse wave number j is incremented by one. At the sametime, a maximum pulse wave level x_(max) is set to zero and a minimumpulse wave level x_(min) is set to an upper limit value x_(sup) which ishigher than a conceivable maximum value of the pulse wave level.

In step 15, it is determined whether the cuff pressure is higher thanlower than a certain predetermined value. If the cuff pressure is higherthan the predetermined value, the process flow advances to step 23 and,after the LED indicating that a measurement process is in progress isturned off in step 23, the process flow returns to step 3 where a fastventing with both the fast vent valves 42 and 43 takes place. However,if the cuff pressure is determined to be lower than the predeterminedvalue, pulse wave data x_(i) is inputted to the MPU 33 in step 16 andthe difference between the current value of the pulse wave data x_(i)and the previous value of the pulse wave data x_(i-1) are compared witha certain value x_(t) in step 17. This step is for defining theboundaries of each cycle of the pulse beat by sudden changes of thepulse wave level.

If this difference is smaller than this certain value x_(t) or, in otherwords, if the determination result of step 17 is negative, the processflow advances to step 24 and the current value of the pulse wave datax_(i) is compared with the maximum pulse wave level x_(max). If x_(i)>x_(max) the maximum pulse wave level x_(max) is updated by the value ofx_(i) in step 24. Conversely, if x_(i) <x_(max) step 25 is skipped andthe process flow advances to step 26. In step 26 the current pulse wavedata x_(i) is compared with the minimum pulse wave level x_(min). Ifx_(i) <x_(min) the minimum pulse wave level x_(min) is updated by thevalue of x_(i) in step 27. Conversely, if x_(i) >x_(min) step 27 isskipped and the process flow advances to step 28.

When the updating of the maximum and minimum pulse wave level x_(max)and x_(min) in steps 24 to 27 is completed, the count i is incrementedby one in step 28 and the process flow returns to step 15. Thereafter,steps 24 to 28 and steps 15 to 17 are repeated and the updating of themaximum and minimum pulse wave level x_(max) and x_(min) continues untilx_(i-1) -x_(i) >x_(t) holds or until the next cycle of pulse beatbegins.

If it is determined that x_(i-1) -x_(i) >x_(t) in step 17, the buzzer 39is activated in step 18 and extraction of the pulse wave is notified tothe user. Then, the difference between the maximum and minimum pulsewave level x_(max) and x_(min) (the amplitude of the pulse wave) A_(j)is derived in step 19 and stored in the MPU 33. And a blood pressuredetermination process is conducted in step 20 using this amplitude A_(j)as a parameter. According to this determination process, the maximumvalue of the pulse wave amplitude A_(j) is extracted and the cuffpressure at the time point at which the maximum pulse wave amplitudeA_(max) is detected is determined as a mean blood pressure P_(mean).Then, the cuff pressure at the time point at which the pulse wavedisappears as the cuff pressure is gradually increased is determined asthe systolic blood pressure P_(sys). And, the diastolic blood pressureP_(dia) a is determined from the following equation:

    P.sub.mean =P.sub.dia +(P.sub.sys -P.sub.dia)/3

And, until the diastolic blood pressure P_(dia) is determined, theprocess flow continues returning from step 21 to step 13 and, afterincrementing the pulse wave number j by one in step 13, the bloodpressure determination process is continued by deriving the differencebetween the maximum and minimum pulse wave level x_(max) and x_(min) orthe amplitude of the pulse wave) A_(j) for each cycle of the pulse beat.

Upon completion of the determination of the systolic and diastolic bloodpressure levels (step 20), these blood pressure values are displayed onthe display unit 19 in step 21 and the LED for indicating that ameasurement process is in progress is turned off in step 23. Thereafter,the process flow returns to step 3 where the fast vent valves 42 and 43are activated again for full venting, and steps 3 to 7 complete themeasurement process.

According to this embodiment, the time required for measurement issubstantially reduced and the discomfort to the patient whose bloodpressure is to be measured can be substantially reduced. Also, theadvantage of this embodiment in reducing in the degree of obstructingblood flow is particularly favorable when the blood pressure of aseriously ill person is to be measured.

FIG. 17 shows yet another embodiment of the present invention whichcomprise a main unit 50 and a cuff unit 51 which are built as separateunits and mutually connected by electric cables 52. A display unit 53 isprovided on the exterior of the cuff unit 51, and a cuff 56 is fittedinto a tubular member 55 provided in the cuff unit 51 and is open on oneend surface of the casing for the cuff unit 51. The cuff 56 itself maybe identical to those described previously.

Although the present invention has been shown and described withreference to the preferred embodiments thereof, it should not beconsidered as limited thereby. Various possible modifications andalterations could be conceived of by one skilled in the art to anyparticular embodiment, without departing from the scope of theinvention.

What we claim is:
 1. A cuff for measuring blood pressure by surroundinga finger of a living body and obstructing the blood in the finger,comprising:an airbag having an internal and an external skin, saidairbag being sized and adapted to surround said finger and having acertain length defined in a direction along said finger and a certainwidth defined in a direction around said finger when said airbag ispositioned on said finger, said internal and external skins having endsand being contiguously joined at said ends along said length and servingto define an inflatable chamber, said air bag width being sufficient forsubstantially surrounding said finger, said internal skin beingpre-formed into a plurality of adjacent sectioned air chambers, each ofsaid sectioned air chambers extending substantially along the entiretyof said airbag length and freely communicating with an adjacent chamberalong the entirety of said length, said internal skin being sufficientlyflexible such that respective sectioned chambers are adapted to beinflated individually with respect to other of said sectioned chamberswhile said airbag is positioned on said finger.
 2. A cuff for measuringblood pressure as defined in claim 1, wherein the internal skin isprovided with a plurality of bulges of a substantially trapezoidal shapedefining the corresponding air chambers, wherein the internal skindefines a shorter side between the two parallel sides of each of thetrapezoidal shaped bulges.
 3. A cuff for measuring blood pressure asdefined in claim 1, wherein the internal skin is provided with aplurality of bulges of a substantially semicircular shape defining thecorresponding air chambers, wherein the internal skin defines a circularportion of each of the semicircular shaped bulges.
 4. A cuff formeasuring blood pressure as defined in claim 2 or 3, wherein theexternal skin is substantially flat.
 5. A cuff for measuring bloodpressure as defined in claim 2 or 3, wherein the external skin issubstantially symmetric to the inner skin in shape.
 6. An electronicblood pressure meter and cuff combination for measuring blood pressureby surrounding a finger of a living body and obstructing the blood inthe finger, comprising:an airbag having an internal and an externalskin, said airbag being sized and adapted to surround said finger andhaving a certain length defined in a direction along said finger and acertain width defined in a direction around said finger when said airbagis positioned on said finger, said internal and external skins havingends and being contiguously joined at said ends along said length andserving to define an inflatable chamber, said air bag width beingsufficient for substantially surrounding said finger, said internal skinbeing pre-formed into a plurality of adjacent sectioned air chambers,each of said sectioned air chambers extending substantially along theentirety of said airbag length and freely communicating with an adjacentchamber along the entirety of said length, said internal skin beingsufficiently flexible such that respective sectioned chambers areadapted to be inflated individually with respect to other of saidsectioned chambers while said airbag is positioned on said finger, atleast one chamber being recessed relative to at least one adjacentchamber for accommodating at least one artery sensor thereon.